1. Field of the Invention
The present invention relates to a process used to refine or smelt zinc sulfide concentrates.
2. Description of the Prior Art
Methods used to obtain zinc metal from zinc sulfide concentrates are broadly divided into hydrometallurgical processes and pyrometallurgical processes.
In both the hydrometallurgical processes and the pyrometallurgical processes for refining zinc, the zinc sulfide concentrates, which are the main raw materials, are first roasted to form zinc oxide. In the hydrometallurgical process, following the roasting the zinc is recovered by acid leaching or electrolytic recovery processes. In the pyrometallurgical process, following the roasting the zinc oxide is charged into a furnace with coke, and the like, and the zinc is recovered by reduction and volatilization.
Only electrolytic refining is used with the hydrometallurgical process, in actual practice. In the electrolytic refining process, the roasted ore obtained by roasting the sulfide ore is dissolved in sulfuric acid to obtain a zinc sulfate solution, then, after removing iron and the like by cleaning the solution, electrolytic zinc is obtained by electrolysis and melted in an electric furnace to obtain zinc metal. However, as moderate as possible a roasting process must be adopted with this process, therefore a fluidized roasting furnace is generally used. For this reason, a zinc concentrate with a high lead content cannot be used because such zinc concentrate is apt to be clustered to form briquettes, and in addition, when the resulting zinc oxide is leached, impurities such as copper, cobalt, nickel, cadmium, and the like are also leached out. Therefore, these impurities must be removed prior to the electrolytic recovery of the zinc.
Pyrometallurgical processes include a horizontal distillation process, a vertical distillation process, an electrothermal distillation process, and an ISP process.
In the horizontal distillation process, the roasted ore and 40 to 60 wt % coal for reducing are mixed together and this mixture is charged into a horizontal retort which is heated from the outide. The zinc is reduced and volatilized, then condensed in a condenser. The horizontal distillation process is a batch process and is therefore extremely labor intensive. The operating environment is also poor, and because this process also offers very few advantages of large scale or mass-production, it has been seldom used since the latter part of the 1970s.
In the vertical distillation process the roasted ore and the like with pulverized coal and powdered coke are kneaded together to form briquettes, which are heated in a carbonizing furnace for coking. The resulting briquettes are heated in a vertical type retort to which heat is supplied from the outside. The retort is fed and heated continuously, so that the zinc is reduced and volatilized from the briquettes, then condensed in a condenser provided on the upper section of the retort. The vertical distillation process utilizes the same principles as the horizontal distillation process, but, whereas the horizontal distillation process has the drawback of poor productivity, the vertical distillation process gives good results in this respect. However, because this process uses a vertical furnace with external heating, the maximum capacity of the furnace is 200 to 300 tons of zinc per month, and the process is highly complicated. It is also necessary to process briquette tails or slags containing copper and lead produced in the furnace, therefore this process is now no longer used to refine zinc.
In the electrothermal distillation process, the roasted ore is mixed with powdered coke and sintered to obtain a sintered ore. This sintered ore is fed into a cylindrical-type furnace and power is applied to vertical electrodes provided in the furnace to subject the mixed raw material to resistance heating in which the raw material itself acts as the resistance, so that the ore is reduced and distilled. The production capacity of the electrothermal distillation process is 1,000 to 3,000 tons of zinc per month, higher than the previously-described two processes. However, the pre-process to obtain the lumps of sintered material which are fed into the furnace is very time consuming. Because an electrically heated furnace is used there is the drawback that there is a limit to the reduction in the electric power consumption rate. Therefore, in regions where the cost of electrical power is high, this process is seldom used.
In the ISP process, the preprocessing comprises mixing the sulfide concentrate with a suitable amount of a solvent, forming a sintered oxide, and removing the sulfur to obtain lumps of sintered material. This sintered material mixed with coke is charged into a blast furnace, then heated and reduced in the blast furnace to volatilize the zinc. Molten lead is splashed through the zinc vapor and the zinc is captured in the form of a lead-zinc alloy. This alloy is then cooled and the zinc and lead solution are separated, utilizing the difference in zinc solubility, and rectified, if required, to obtain zinc metal. The ISP process has the special feature of simultaneous smelting of the zinc and the lead, and is the main pyrometallurgical process in present day use.
The ISP process has been widely adopted from among the pyrometallurgical processes because the productivity of the ISP process is high, it can provide simultaneous smelting of the zinc and the lead, and the allowable amount of impurities is high.
In the ISP process, zinc sulfide concentrates are roasted or sintered together with lead concentrates or zinc concentrates containing lead, to obtain a sintered ore with adequate strength. Technology has been developed and adopted for the ISP process by which even in an atmosphere rich in carbon dioxide gas which has a reoxidizing tendency, the gas containing zinc vapor can be processed at a high temperature of 1,000.degree. C. or greater in a molten lead splash condenser to condense zinc. Accordingly, the production volume for one furnace is increased as high so 6,000 to 10,000 tons of zinc per month.
The ISP process can, in fact, be said to have many advantages in productivity, thermal efficiency, and raw material handling, but to obtain the sintered lumps to feed to the blast furnace, it is impossible to avoid the repeated recycling of powder in the roasting and sintering processes equivalent to about four times the ore. Furthermore, the operation of the above-mentioned roasting and sintering processes requires skill, and high priced lump coke are required for the blast furnace.
Furthermore, if the roasting temperature is set rather high to promote oxidation in the sulfur removal process which is a preprocess for the ISP process, part of the raw material melts, fuses and sticks to the roasting equipment, making it difficult to discharge the roasted material from this equipment. In the worst case, it becomes necessary to halt the process of whole operation. In addition, cohesion of the particles occurs because part of the raw material melts, and the surface area of the reacting particles decreases in size so that the roasting temperature must be reduced to below 1,100.degree. C., which in turn decreases the rate of sulfur removal. Even at a roasting temperature of 1,100.degree. C. or less, the equivalent of about four times the raw material fed into the roasting equipment must normally be recycled as returned powder to prevent cohesion of the particles. In addition, the problem occurs that when the roasting temperature is lowered, the effective utilization of the heat of oxidation produced in the desulfurizing reaction is not realized.
A report relating to a oxidizing reaction for zinc sulfide appears in Metallurgical Transactions B (Voume 21B; October 1990; pp. 867 to 872). In this process, the ZnS is first embedded in slag and reacts with the FeO in the slag. And a lance is inserted into the slag for oxygen at this time. As a result, a reaction between ZnS and O.sub.2 takes place within the slag. Accordingly, the reaction of this report differs from a reaction in a production scale reaction furnace into which zinc sulfide and O.sub.2 are added from above the slag bath.